This project is designed to investigate the ecological importance of coaggregation between oral bacteria. These cell to cell interactions appear to be mediated by complementary surface components composed of a lectin on one cell type and a carbohydrate receptor on the other cell type. Mutants have been isolated and used to prove the fimbrial structures on the surface of Actinomyces viscosus T14V possess the lactose-sensitive lectin activity that mediates lactose-reversible coaggregation with Streptococcus sanguis 34. Mutants of A. naeslundii ATCC 12104 and of S. sanguis H1 have also been isolated and are being used to characterize other surface components involved in coaggregation. Lactose-reversible coaggregations between the gram negative Capnocytophaga ochracea and A. naeslundii, A. israelii, or S. sanguis have been discovered which suggests that this type of coaggregation may be widespread among oral bacteria. Analysis of coaggregation between fresh human oral isolates of S. sanguis and A. viscosus or A. naeslundii obtained from the same site revealed that almost all isolates of these three species found in plaque are capable of coaggregation. Further study of interactions between oral bacteria is expected to provbide a better understanding of the ecology of these bacteria in the subgingival ecological niche.